encoding/protobuf: remove unused code

When refactoring cue/load, I noticed that some pieces of code
were unused, which makes it less obvious what kinds of things
can be done. I used `staticcheck` to point out many other
places in the code that are unused.

This is one of those places. I feel it's better to remove this code
even if it might be used in the future (it's always there to be found
in the git history).

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I4b2df1c3b2dd6d6dc521ca3116e19fd3fab6a71e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/549293
Unity-Result: CUEcueckoo <cueckoo+gerrithub@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo+gerrithub@cuelang.org>